How do I relax my body to be sensitive for chi sao after a workout?

I am a mantis practitioner, and I am always told to be light, fast, yet powerful at the same time. After doing workout, forms, my muscles tend to be tight and shake, and i have a hard time being light, sensitive. My instructor is always telling me to loosen up, and i don't know how since i am exhausted from the workout. What some good exercises that I can do in order to accomplish this feat that is kicking my .